To reproduce (in 1.8.3 and 1.9, probably earlier versions as well):
Create a quiz with essay questions.
Log in as a student, take the quiz, apply some formatting to the essay answer in the html editor, and submit.
As a teacher, go to the Item Analysis tab and download the report (any format).
The student response in the downloaded report will contain html markup. Some of the column headers also contain <br /> tags, and question names and text can contain markup as well. This makes the downloaded reports difficult for faculty to analyze.
When downloading reports, the html should first be converted to text. A patch is attached (1.9).
Note that the same issue exists with the Detailed Responses plugin. I'll file a separate report for that one.